1. Field of Invention
This invention relates to a Fabry-Perot filter device used as a wavelength selective filter for transmitting rays of light, such as infrared radiation, in a wavelength selective manner; a wavelength selective infrared detector comprising the Fabry-Perot filter device and an infrared detector; and a non-dispersive infrared gas analyzer for measuring concentration of a gas or gases by detecting infrared radiation transmitted through the Fabry-Perot filter device.
2. Description of the Prior Art
A non-dispersive infrared gas analyzer, hereinafter called xe2x80x9cNDIR gas analyzerxe2x80x9d, is used to analyze a gas, wherein the analyzer detects the amount of infrared absorption to measure the concentration of the gas,utilizing the principle that the wavelength bands of infrared radiation to be absorbed depends on the type of gas.
FIGS. 1 to 5 show examples of prior art NDIR gas analyzers. In the following, the gas being measured is assumed to be carbon dioxide whose peak wavelength of infrared radiation absorption is approximately 4.25 xcexcm.
FIG. 1 shows a single ray, one wavelength NDIR gas analyzer comprising a sample cell 110, whereinto a gas is supplied; a light source 111; a filter 112; and an infrared detector 113. In this example, filter 112 is tuned to the absorption bands of carbon dioxide shown in FIG. 6 to select and pass a band of infrared radiation having wavelengths in the vicinity of 4.25 xcexcm. Infrared detector 113 determines the concentration of the gas being measured by detecting infrared radiation transmitted through filter 112.
FIG. 2 shows a single ray two wavelength comparative NDIR gas analyzer, wherein the analyzer selects two wavelength bands using filter 112, tuned to the absorption band of carbon dioxide, and a filter 114 for transmitting an infrared radiation band of wavelengths in the vicinity of approximately 3.9 xcexcm, as a reference light. The bands of infrared radiation thus selected are detected respectively with infrared detectors 113 and 115. In this example, it is possible to correct a change in the output signal with time due to, for example, deterioration in light source 111 or contamination in sample cell 110 by comparing the carbon dioxide absorption band with the measured absorption band of the reference light.
FIG. 3 shows another single ray two wavelength comparative NDIR gas analyzer, wherein the analyzer selects two wavelength bands using filter 112 formed on a disc 116 and tuned to the absorption band of carbon dioxide and the filter 114 for reference light. The bands of infrared radiation selected by the filters as disc 116 is rotated are then detected with infrared detector 113. In this example, it is also possible to correct a change in the output signal with time due to, for example, deterioration in light source 111 or contamination in sample cell 110, by comparing the carbon dioxide absorption band with the measured absorption band of the reference light.
FIG. 4 shows a single ray two wavelength Fabry-Perot NDIR gas analyzer, wherein a gap between two parallel mirrors, comprising a Fabry-Perot filter 117, is made variable so that two bands are selected, one being a band tuned to the absorption band of the gas being measured and the other being a band of reference light. The bands of infrared radiation thus selected are respectively detected with infrared detector 113. In this example, it is also possible to correct a change in the output signal with time due to, for example, deterioration in light source 111 or contamination in sample cell 110, by comparing the carbon dioxide absorption band with the measured absorption band of the reference light.
As shown in FIG. 8, the Fabry-Perot filter is an optical filter using a device (e.g. Fabry-Perot plates) wherein a pair of high reflectance mirrors 120 and 121 are placed in parallel and opposite to each other with a gap formed therebetween. Assuming that the width (i.e. distance between) of the gap in the Fabry-Perot filter is xe2x80x9cdxe2x80x9d and the refractive index within the gap is xe2x80x9cnxe2x80x9d, then the rays, among the rays of incident light, that satisfy the phase relationship represented by below equation (1) will enhance each other by interference, thus becoming transmitted light. In equation (1), xcex4 is the phase reference, xcfx86 is the angle of light incident on the device, and xcex is the wavelength of the light. FIG. 9 shows how the rays are transmitted.
xcex4=4¶nd cosxcfx86/xcexxe2x80x83xe2x80x83(1) 
The Fabry-Perot filter is designed so that the wavelength bands of the transmitted light passing through the device can be varied by making the gap width xe2x80x9cdxe2x80x9d variable. An example of a variable wavelength Fabry-Perot filter comprises a fixed mirror, and a movable mirror arranged opposite to the fixed mirror with a gap formed therebetween, with a fixed electrode on the fixed mirror and a movable electrode on the movable mirror, and means are provided for applying a voltage to the electrodes so that the movable mirror is moved accordingly and the gap is thereby adjusted in width. For example, such a Fabry-Perot filter can be used as a wavelength selective filter for a non-dispersive infrared carbon dioxide sensor and selectively pass the wavelength band of carbon dioxide absorption, of approximately 4.25 xcexcm, and the wavelength band of reference light, of approximately 3.9 xcexcm.
FIG. 5 shows a double ray, one wavelength NDIR gas analyzer, wherein two light sources 111 and 118 are arranged so that the optical paths thereof differ from each other within sample cell 110. In this example, a change in the output signal with time due to, for example, contamination of sample cell 110 is corrected according to the ratio between two output signals of infrared detector 113 provided by a band of infrared radiation that is transmitted through filter 112 tuned to the absorption band of the gas being measured. The concentration of the gas being measured is then determined.
In the foregoing type of NDIR gas analyzer, however, the number of filters must be increased to be able to measure concentrations of a plurality of gases or a plurality of constituents of a gas. Hence, the prior art gas analyzers all have such problems as high cost and large sizes.
Accordingly, an object of the invention is to overcome the aforementioned and other problems, deficiencies and disadvantages of the prior art.
Another object is to provide an infrared gas analyzer capable of simultaneously determining concentrations of at least two constituents of a gas being measured by using a Fabry-Perot filter device for selectively transmitting at least three wavelength bands of infrared radiation which includes a reference light.
The foregoing and other objects are attained in the invention which encompasses a Fabry-Perot filter device for passing infrared radiation from a light source in a wavelength selective manner, and comprising a fixed mirror formed on a substrate; a movable mirror arranged opposite to the fixed mirror with a gap formed therebetween so that the movable mirror is displaced with respect to the fixed mirror by applying an external force; a fixed electrode formed on the fixed mirror; and a movable electrode formed on the movable mirror and arranged opposite to the fixed electrode; wherein the movable mirror is displaced by applying a potential difference across the fixed and movable electrodes so that the width of the gap is varied in at least three steps, whereby at least three wavelength bands of the infrared radiation are selectively transmitted through the filter device.
A feature of the invention is the fixed mirror and the movable mirror both comprising silicon.
Another feature is the fixed electrode and the movable electrode both comprise silicon with a high impurity concentration.
Another aspect of the invention is an infrared gas analyzer for determining concentration of a gas being measured according to output of an infrared detector, end comprising a light source for emitting infrared radiation to the gas being measured; a wavelength selective filter for passing the infrared radiation from the light source in a wavelength selective manner; and an infrared detector for detecting infrared radiation passing through the wavelength selective filter, wherein the wavelength selective filter comprises the above referred to Fabry-Perot filter device.
Another feature of the invention is that the wavelength selective filter is disposed before the infrared detector in an optical path and comprises a wide bandpass filter for passing only a specific band of wavelengths.
A further aspect of the invention is a wavelength selective infrared detector comprising a substrate; a wavelength selective filter formed on the substrate for passing infrared radiation from a light source in a wavelength selective manner; and an infrared detector formed on the substrate for detecting infrared radiation passing through the wavelength selective filter.
A feature of the invention is that the infrared detector comprises elements located in a sealed cavity formed in the substrate, and wherein the wavelength selective filter is formed on the sealed cavity so that the sealed cavity is sealed with an inert gas therein.
A yet further aspect of the invention is a wavelength selective infrared detector comprising a wavelength selective filter formed on a first substrate to pass infrared radiation from a light source in a wavelength selective manner; and an infrared detector formed on a second substrate and comprising detector elements for detecting infrared radiation passing through the wavelength selective filter, wherein the first and second substrates are joined together.
A further feature is that in the just mentioned wavelength selective infrared detector, the infrared detector elements are located within a grooved shaped part formed in the second substrate so as to form a sealed cavity and the wavelength selective filter is positioned on the groove shaped part to form the sealed cavity, so that the sealed cavity has inert gas therein.
Another feature is that in the just mentioned wavelength selective infrared detector, the wavelength selective filter is a Fabry-Perot filter device comprising a fixed mirror located on a substrate and a movable mirror arranged opposite to the fixed mirror so that a gap is formed therebetween and the movable mirror is displaced with respect to the fixed mirror by applying an external force.
A further feature is that a fixed electrode is formed on the fixed mirror and a movable electrode is formed on the movable mirror and electric power is applied to the electrodes to cause the movable mirror to be displaced and the width of the gap to be varied.
Another feature is that the gap is varied by applying a plurality of voltages across the fixed electrode and movable electrodes so that a plurality of bands of wavelengths are passed by the filter device.
A yet further feature is that the infrared detector of the invention may comprise a bolometer.
A still further feature is a plurality of the wavelength selective filters and a plurality of infrared detectors arranged horizontally in arrays.
A further aspect of the invention is an infrared gas analyzer for determining concentration of a gas being measured according to output of an infrared detector, comprisng a light source for emitting infrared radiaton to the gas being measured; a wavelength selective filter for passing infrared radiation from the light source in a wavelength selective manner; and an infrared detector for detecting infrared radiation passing through the wavelength selective filter; wherein the wavelength selective filter is formed on a substrate, and the infrared detector is formed on the same substrate or the infrared detector is formed on a second substrate with the second substrate being joined to the first substrate.
A feature of the invention is a wide bandpass filter being disposed between the wavelength selective infrared detector and the light source so as to pass only a specific band of wavelength.
Another feature of the invention is that the gas being measured contains one component of carbon dioxide, and another component being water, or carbon monoxide, or nitrogen oxide, or sulfur oxide, and wherein concentrations thereof are measured.
A further feature is that the gas being measured is nitrogen oxide and sulfur oxide.
A further aspect of the invention comprises a Fabry-Perot filter device comprising a fixed mirror and a movable mirror with a gap formed therebetween, and wherein the movable mirror is formed of a multilayer optical thin film comprising at least one layer presenting tensile stress; or at least one layer presenting compressive stress, or at least two layers presenting different levels of tensile stress, or at least three layers comprising a high refractive index layer, a low refractive index layer, and another high refractive index layer, of different tensile and compressive stress.
A feature of the invention is that the thin film has a thickness of xcex/4 wherein xcex is the wavelength.
Another feature is that an isolation layer is provided between a fixed electrode which is formed on the fixed mirror and a movable electrode which is formed on the movable mirror.
Another feature is that the isolation layer is formed on the fixed electrode and is made of silicon nitride or silicon oxide.
A further feature is that the fixed mirror is formed on a substrate, and the gap is formed by depositing a sacrificial layer of predetermined shape and size between the fixed mirror and the movable mirror and then removing the sacrificial layer completely or partially, such as by means of etching.
A yet further feature is that the artificial layer has a trapezoid shaped cross section and electrode pads are formed outside of said sacrificial layer.
Another feature is that an anti-refection layer is formed on a backside of a substrate, and a metal aperture is formed on the anti-reflection layer through a protection layer and having an optical area in part, wherein the sacrificial layer is first removed by etching and then a portion of the protection layer present in the optical area is removed.
A further feature is that etching holes are provided at a center and along a periphery of the movable mirror in order to etch the sacrificial layer.